Lightweight particles and compositions containing them

ABSTRACT

Disclosed are particles that have an exterior surface coated with a thin polymeric coating, such as a coating that includes a sulfur-containing polymer. Also disclosed are compositions, such as fuel-resistant sealant and coating compositions, which include such particles. Aerospace vehicles having an aperture at least partially sealed with a sealant deposited from such a sealant composition are also disclosed.

FIELD OF THE INVENTION

The present invention is directed to particles, such as microparticles,and compositions, such as coating and sealant compositions, whichcomprise such particles.

BACKGROUND OF THE INVENTION

Many articles have surfaces that are coated with one or more protectiveand/or decorative coatings for any of a variety of reasons. Aerospacevehicles constitute one exemplary category of such articles. Varioustypes of coatings, such as primers and topcoats are commonly applied tovarious surfaces of aerospace vehicles, such as airplanes orhelicopters, as well as many other types of articles. These coatingsperform a variety of protective and decorative functions.

In addition, many articles have apertures that may exist, for example,between two components of the article. A sealant is often used to sealsuch apertures and aerospace vehicles constitute an exemplary categoryof such articles. Sealants are often used in the manufacture of aircraftfor, among other things, integrated fuel tanks and fuel cell cavities.As will be appreciated, such sealants must be resistant to swelling uponprolonged exposure to fuels typically used in such tanks. Otherdesirable properties for such sealants include high tensile strength,low temperature flexibility, and liquidity at room temperature.

Recently, aircraft manufacturers, in an effort to increase the fueleconomy and lifespan of their airplanes, have sought ways to reduceaircraft weight. As a matter of fact, it is believed that even a slightreduction in weight, even as little at 10 pounds, can save hundreds ofthousands of dollars over the lifespan of certain aircraft.

As a result, coating and sealant manufacturers have sought toincorporate lightweight fillers into their formulations to reduce thedensity of certain of their products. A problem, however, has been thatonly a relatively small amount of such fillers can be added to certainformulations, such as fuel-resistant sealant formulations, withoutdramatically negatively impacting upon certain properties. One problemassociated with these lightweight fillers is that they may exhibit poorchemical resistance to organic solvents, such as fuels to which acomposition may be exposed when applied to certain portions of anaircraft. As a result, the incorporation of such lightweight fillers hasbeen limited such that the specific gravity of aerospace sealants, byway of example, has only been successfully reduced from the 1.6 to 1.8range to a minimum range of approximately 1.0 to 1.2 by virtue of thelimited inclusion of such fillers. In addition to fuel resistance, thelightweight fillers can also have a negative impact upon desirablephysical properties, such as tensile strength and/or elongation.

Therefore, it would be desirable to provide improved lightweight fillerssuitable for use in various compositions, such as coatings and sealants,including, but not limited to, aerospace coating and sealantcompositions. In particular, it would be desirable to providelightweight fillers that can be incorporated into an aerospace sealantcomposition in an amount sufficient to provide a sealant having aspecific gravity of, for example, 0.9 or less, without dramaticallynegatively impacting upon at least the tensile strength, elongation, andfuel resistance properties of the sealant. The present invention hasbeen developed in view of the foregoing desire.

SUMMARY OF THE INVENTION

In certain respects, the present invention is directed to particlescomprising an exterior surface coated with a thin coating, wherein thincoating comprises the reaction product of: (a) an aminoplast resin; and(b) a compound comprising functional groups reactive with the aminoplastresin.

In other respects, the present invention is directed to particlescomprising an exterior surface coated with a thin coating, wherein thethin coating comprises a sulfur-containing polymer.

In still other respects, the present invention is directed tocompositions, such as coating and sealant compositions. Thesecompositions of the present invention comprise: (1) a polymericfilm-forming binder comprising reactive functional groups; (2)optionally a crosslinking agent having functional groups reactive withthe functional groups of the polymeric film-forming binder; and (3)particles dispersed in the binder, wherein the particles comprise anexterior surface coated with a thin coating comprising a polymercomprising reactive functional groups that are reactive with thefunctional groups of (1) and/or (2).

In yet other respects, the present invention is directed to compositionsthat comprise: (a) a sulfur-containing polymer binder; and (b)lightweight particles dispersed in the binder. These compositions have aspecific gravity of less than 1.0, such as no more than 0.9, or, in somecases, no more than 0.85 g/cm³. Moreover, these compositions, whenapplied to a substrate and cured, result in a cured composition, such asa cured sealant, that has: (i) a percent volume swell of not greaterthan 40%, in some cases not greater than 25%, in some cases not greaterthan 20% after immersion for one week at 140° F. (60° C.) and ambientpressure in jet reference fluid (JRF) type 1; (ii) an elongation of atleast 80% when measured as described in AMS 3269a; and (iii) a tensilestrength of at least 2000 mega-Pascals when measured according to AS5127/1a 7.7.

The present invention is also directed to aerospace vehicles having anaperture at least partially sealed with a sealant deposited from asealant composition of present invention, as well as related methods.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 a and 1 b illustrate particles in accordance with certainembodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

For purposes of the following detailed description, it is to beunderstood that the invention may assume various alternative variationsand step sequences, except where expressly specified to the contrary.Moreover, other than in any operating examples, or where otherwiseindicated, all numbers expressing, for example, quantities ofingredients used in the specification and claims are to be understood asbeing modified in all instances by the term “about”. Accordingly, unlessindicated to the contrary, the numerical parameters set forth in thefollowing specification and attached claims are approximations that mayvary depending upon the desired properties to be obtained by the presentinvention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard variation found in theirrespective testing measurements.

Also, it should be understood that any numerical range recited herein isintended to include all sub-ranges subsumed therein. For example, arange of “1 to 10” is intended to include all sub-ranges between (andincluding) the recited minimum value of 1 and the recited maximum valueof 10, that is, having a minimum value equal to or greater than 1 and amaximum value of equal to or less than 10.

As indicated, certain embodiments of the present invention are directedto particles, such as microparticles and/or nanoparticles. As usedherein, the term “microparticle” refers to small particles with adiameter that falls essentially in the micrometer range, i.e., 1 micronor larger. As used herein, the term “nanoparticle” refers to smallerparticles with a diameter that falls essentially in the nanometer range,i.e., less than 1 micron, such as 0.1 to 500 nanometers, 0.1 to 300nanometers, 0.1 to 100 nanometers, or, in some cases, 0.1 to 50nanometers. As used herein, the general term particles encompasses bothmicroparticles and nanoparticles.

The shape (or morphology) of the particles can vary. For example,generally spherical morphologies (such as solid beads, microbeads, orhollow spheres), can be used, as well as particles that are cubic,platy, or acicular (elongated or fibrous). Additionally, the particlescan have an internal structure that is hollow, porous or void free, or acombination of any of the foregoing, e.g., a hollow center with porousor solid walls. For more information on suitable particlecharacteristics see H. Katz et al. (Ed.), Handbook of Fillers andPlastics (1987) at pages 9-10.

Exemplary, but non-limited, particles that are suitable for use in thepresent invention are described in U.S. Patent Application PublicationNo. 2006/0252881 A1 at paragraphs [0028] to [0055], the cited portion ofwhich being incorporated herein by reference.

In certain embodiments, however, the particles of the present inventionare lightweight particles. As used herein, the term “lightweight” whenused with reference to a particle of the present invention means thatthe particle, prior to deposition of a thin coating as described herein,has a specific gravity of no more than 0.7, in some cases no more than0.25 or no more than 0.1. Suitable lightweight particles of the presentinvention often fall within two categories—microspheres and amorphousparticles. The specific gravity of the microspheres often ranges from0.1 to 0.7 and include, for example, polystyrene foam, microspheres ofpolyacrylates and polyolefins, and silica microspheres having particlesizes ranging from 5 to 100 microns and a specific gravity of 0.25(ECCOSPHERES®, W. R. Grace & Co.). Other examples include alumina/silicamicrospheres having particle sizes in the range of 5 to 300 microns anda specific gravity of 0.7 (FILLITE®, Pluess-Stauffer International),aluminum silicate microspheres having a specific gravity of from about0.45 to about 0.7 (Z-LIGHT®), and calcium carbonate-coatedpolyvinylidene copolymer microspheres having a specific gravity of 0.13(DUALITE 6001AE®, Pierce & Stevens Corp.).

In certain embodiments of the present invention, the particles comprisethermally expandable capsules. As used herein, the term “thermallyexpandable capsule” refers to a small hollow shell comprising a volatilematerial that expands at a predetermined temperature. In certainembodiments, such thermally expandable capsules have an average initialparticle size of 5 to 70 μm, in some cases 10 to 24 μm, and in yet othercases, 10 to 17 μm. As used herein, the term “average initial particlesize” refers to the average particle size of the capsules prior to anyexpansion.

In certain embodiments, the thermally expandable capsule comprises avolatile hydrocarbon positioned within a wall of a resin, such as athermoplastic resin. Examples of hydrocarbons suitable for use in suchcapsules are, without limitation, methyl chloride, methyl bromide,trichloroethane, dichloroethane, n-butane, n-heptane, n-propane,n-hexane, n-pentane, isobutane, isopentane, iso-octane, neopentane,petroleum ether, and aliphatic hydrocarbons containing fluorine, such asFreon, or a mixture thereof.

Examples of the materials which are suitable for forming the wall of thethermally expandable capsule are, without limitation, polymers ofvinylidene chloride, acrylonitrile, styrene, polycarbonate, methylmethacrylate, ethyl acrylate, and vinyl acetate, copolymers of thesemonomers, and mixtures of the polymers of the copolymers. A crosslinkingagent may be used if desired.

Thermally expandable capsules suitable for use in the present inventionare commercially available from various companies, specific examples ofwhich include Union Carbide Corporation's Ucar and PhenolicMicroballoons (phenol balloons), Emerson & Cuming Company's Eccospheres(epoxy balloons), Emerson & Cuming Company's Eccospheres VF-O (ureaballoons), Dow Chemical Company's Saran Microspheres, AKZO NOBEL'sExpancel and Matsumoto Yushi Seiyaku Co., Ltd.'s Matsumoto Microspheres(Saran balloons), Arco Polymers Inc.'s Dylite Expandable Polystyrene andBASF-Wyandotte's Expandable Polystyrene Beads (polystyrene balloons),and JSR Corporation's SX863(P) (crosslinked styrene-acrylic balloons).

As previously indicated, the particles of the present invention comprisean exterior surface having a thin coating deposited thereon. Referringnow to FIGS. 1 a and 1 b, there are depicted certain embodiments of theparticles of the present invention. As is apparent, in each case, theparticle 10 comprises an outer surface 20 upon which is deposited a thinfilm 30. In certain embodiments, such as is depicted in FIG. 1 a, theouter surface of the particle is simply the exterior portion of a solid,i.e., non-hollow, particle. In other embodiments, such as is depicted inFIG. 1 b, the exterior surface is the outer surface of a thin wall 25,such as is the case with a hollow particle which may or may not comprisea volatile liquid therein.

In the present invention, a thin coating covers at least a portion ofthe exterior surface of the particle. In certain embodiments, thecoating is a substantially continuous coating covering from 70 to 100,80 to 100, 90 to 100, or 100 percent of the entire surface area of theparticle. In certain embodiments, the thin coating has a film thicknessof less than 25, 20, 15, or 5 micrometers. In certain embodiments, thethin coating has a film thickness of at least 0.1 nanometers, such as atleast 10 nanometers, or at least 100 nanometers, or, in some cases, atleast 500 nanometers.

The particles of the present invention are distinct from situationswhere particles are merely encapsulated throughout a polymer network,such as is the case when particles are dispersed in a film-formingbinder. In the present invention, a thin film is deposited on theexterior surface of an individual discrete particle. These resultingcoated particles may then subsequently be dispersed in a film-formingbinder, thereby resulting in dispersion of the coated particlesthroughout a polymer network.

As indicated, in certain embodiments of the particles of the presentinvention an exterior surface of the particle is coated with a thincoating comprising: (1) an aminoplast resin; and (2) a compoundcomprising functional groups reactive with the aminoplast resin.

In certain embodiments of the particles of the present invention, thethin coating present on the exterior surface of the particle comprises asulfur-containing polymer. As used herein, the term “polymer” includesoligomers and both homopolymers and copolymers, and the prefix “poly”refers to two or more. As used herein, the term “sulfur-containingpolymer” refers to any polymer having at least one sulfur atom,including, but not limited to, polymeric thiols, polythiols, thioethers,polythioethers and polysulfides. A “thiol”, as used herein, refers to acompound comprising a thiol or mercaptan group, that is, an “SH” group,either as the sole functional group or in combination with otherfunctional groups, such as hydroxyl groups, as is the case with, forexample, thioglycerols. A “polythiol” refers to a compound having morethan one SH group, such as a dithiol or higher functionality thiol. Suchgroups are typically terminal and/or pendent such that they have anactive hydrogen that is reactive with other functional groups. A“thioether” or “polythioether” refers to a compound that contains one ormore sulfur atoms, respectively, such as within the backbone of apolymer, that do not contain an active hydrogen group; that is, they arebonded on either side to another sulfur atom, a carbon atom, and thelike. As used herein, the term “polysulfide” refers to any compound thatcomprises a sulfur-sulfur linkage (—S—S—). A “polythiol” can compriseboth a terminal and/or pendant sulfur (—SH) and a non-reactive sulfuratom (—S— or (—S—S—)). Thus, a “polythiol” is often also a“polythioether” and/or a “polysulfide”. Suitable polythiols include, forexample, those disclosed in U.S. Pat. No. 7,009,032, incorporated byreference herein.

In certain embodiments of the particles of the present invention, thethin coating deposited on the exterior surface of the particle comprisesa sulfur-containing polymer that is the reaction product of anaminoplast resin and a polythiol.

The aminoplast resins suitable for use in the preparation of particlesof the present invention include those which are or are derived from atleast one of glycoluril, aminotriazine and benzoguanamine. Suchcompounds include, for example, alkoxyalkyl derivatives of melamine,glycoluril, benzoguanamine, acetoguanamine, formoguanamine,spiroguanamine, and the like.

Aminoplast resins are based on the condensation products offormaldehyde, with an amino- or amido-group carrying substance.Condensation products obtained from the reaction of alcohols andformaldehyde with melamine, urea or benzoguanamine are most common.However, condensation products of other amines and amides can also beemployed, for example, aldehyde condensates of triazines, diazines,triazoles, guanadines, guanamines and alkyl- and aryl-substitutedderivatives of such compounds, including alkyl- and aryl-substitutedureas and alkyl- and aryl-substituted melamines. Some examples of suchcompounds are N,N′-dimethyl urea, benzourea, dicyandiamide,formaguanamine, acetoguanamine, glycoluril, ammeline,2-chloro-4,6-diamino-1,3,5-triazine,6-methyl-2,4-diamino-1,3,5-triazine, 3,5-diaminotriazole,triaminopyrimidine, 2-mercapto-4,6-diaminopyrimidine and3,4,6-tris(ethylamino)-1,3,5 triazine.

While the aldehyde employed is often formaldehyde, other similarcondensation products can be prepared from other aldehydes such asacetaldehyde, crotonaldehyde, acrolein, benzaldehyde, furfural andglyoxal.

The aminoplast resins can contain methylol or other alkylol groups, andin most instances, at least a portion of these alkylol groups areetherified by a reaction with an alcohol. Any monohydric alcohol can beemployed for this purpose, including alcohols such as methanol, ethanol,propanol, butanol, pentanol, hexanol, heptanol and others, as well as,benzyl alcohol and other aromatic alcohols, cyclic alcohols such ascyclohexanol, monoethers of glycols, and halogen-substituted or othersubstituted alcohols, such as 3-chloropropanol and butoxyethanol.Commonly employed aminoplast resins include those substantiallyalkylated with methanol or butanol.

In certain embodiments of the present invention, the aminoplast resincomprises highly alkylated, low imino aminoplast resins which have adegree of polymerization (“DP”) of less than 3.75, often less than 3.0,and, in some cases, less than 2.0. Generally, the number average degreeof polymerization is defined as the average number of structural unitsper polymer chain (see George Odian, Principles of Polymerization, JohnWiley & Sons (1991)). For purposes of the present invention, forexample, a DP of 1.0 would indicate a completely monomeric triazinestructure, while a DP of 2.0 indicates two triazine rings joined by amethylene or methylene-oxy bridge. It should be understood that the DPvalues reported herein and in the claims represent average DP values asdetermined by gel permeation chromatography data.

Non-limiting examples of suitable aminotriazine compounds includealkoxyalkyl aminotriazines, such as (methoxymethyl)melamine-formaldehyderesin, for example, RESIMENE® CE-7103, 745, and 747 commerciallyavailable from Solutia, Inc. and CYMEL® 300, 303; ethylated-methylatedbenzoguanimine-formaldehyde resin, for example CYMEL® 1123;ethylated-methylated melamine-formaldehyde resin, for example CYMEL®1116; and methylated-butylated melamine-formaldehyde resin, for exampleCYMEL® 202, 235, 238, 254, 272, 1135, 1133, 1168 commercially availablefrom Cytec Industries, Inc and RESIMENE® 755, 757 commercially availablefrom Solutia, Inc.

As indicated, in accordance with certain embodiments of the presentinvention, an aminoplast is reacted with a compound comprisingfunctional groups reactive therewith to form the thin wall film that isdeposited on the exterior surface of the particles of the presentinvention. Suitable functional groups include, without limitation,hydroxyl and thiol groups. As will be appreciated, such groups arereactive with alkylol groups present in the aminoplast resin. In certainembodiments, a polythiol is employed. In these embodiments, the relativeamounts of the reactants can be selected so as to result in an aminofunctional sulfur-containing polymer or a thiol-functionalsulfur-containing polymer, such as a thiol functional polysulfide orpolythioether. As a result, in certain embodiments, for example, (>n)moles of a polythiol, such as a dithiol having the structure (I):HS—R¹—SH   (I)or a mixture of at least two different compounds having the structure(I), are reacted with (n) moles of an aminoplast to provide a thiolfunctional sulfur-containing polymer as the thin coating on theparticles of the present invention.

In certain embodiments, such dithiols include those compounds in whichR¹ is a C₂₋₆ n-alkylene group, i.e., 1,2-ethanedithiol,1,3-propanedithiol, 1,4-butanedithiol, 1,5-pentanedithiol or1,6-hexanedithiol.

Other suitable dithiols include those compounds in which R¹ is a C₃₋₆branched alkylene group, having one or more pendent groups which can be,for example, methyl or ethyl groups. Suitable compounds in which R¹ instructure (I) is a branched alkylene group include 1,2-propanedithiol,1,3-butanedithiol, 2,3-butanedithiol, 1,3-pentanedithiol and1,3-dithio-3-methylbutane. Other useful dithiols include those in whichR¹ is a C₆₋₈ cycloalkylene or C₆₋₁₀ alkylcycloalkylene group, forexample, dipentenedimercaptan and ethylcyclohexyldithiol (ECHDT).

Further suitable dithiols include one or more heteroatom substituents inthe carbon backbone, that is, dithiols in which X (described below) is aheteroatom such as O, S or another bivalent heteroatom radical; asecondary or tertiary amine group, i.e., —NR—, where R is hydrogen ormethyl; or another substituted trivalent heteroatom. In certainembodiments, X is O or S, and R¹ in structure (I) is:—[(—CHR³—)_(s)—O—]_(q)—(—CHR³—)_(r)—, or—[(—CHR³—)_(s)—S—]_(q)—(—CHR³—)_(r)—where R³ is hydrogen or an alkyl, such as a methyl, group, s is aninteger having a value ranging from 2 to 6, q is an integer having avalue ranging from 1 to 5, and r is an integer having a value rangingfrom 2 to 10. In certain embodiments, the indices s and r are equal,and, in some cases, both have the value of 2. Exemplary dithiols of thistype include dimercaptodiethylsulfide (DMDS) (s, r=2, q=1, X═S, R³═H);dimercaptodioxaoctane (DMDO) (s, q, r=2, X═O, R³═H); and1,5-dimercapto-3-oxapentane (s,r=2, q=1, X═O, R³═H). It is also possibleto employ dithiols that include both heteroatom substituents in thecarbon backbone and pendent alkyl, such as methyl, groups. Suchcompounds include methyl-substituted DMDS, such as:HS—CH₂CH(CH₃)—S—CH₂CH₂—SHandHS—CH(CH₃)CH₂—S—CH₂CH₂—SHand dimethyl substituted DMDS, such as:HS—CH₂CH(CH₃)—S—CH(CH₃)CH₂—SHandHS—CH(CH₃)CH₂—S—CH₂CH(CH₃)—SH.Two or more different dithiols of structure (I) can also be employed ifdesired.

In certain embodiments, the polythiol that is reacted with theaminoplast comprises a polymeric polythiol, such as a thiol functionalpolysulfide or polythioether. In certain embodiments, the polythiolcomprises a thiol-terminated polythioether, such as, for example, thosehaving the following structure (II):HS—R¹—[—S—(CH₂)_(p)—O—[—R²—O—]_(m)—(CH₂)₂—S—R¹—]_(n)—SH   (II)wherein R¹ in structure (II) denotes a C₂₋₁₀ n-alkylene group, such as aC₂₋₆ n-alkylene group; a C₂₋₆ branched alkylene group, such as a C₃₋₆branched alkylene group having one or more pendant groups which can be,for example, alkyl groups, such as methyl or ethyl groups; analkyleneoxy group; a C₆₋₈ cycloalkylene group; a C₆₋₁₀alkylcycloalkylene group; a heterocyclic group; or—[(—CHR³—)_(s)—X—]_(q)—(—CHR³—)_(r)—, wherein s is an integer having avalue ranging from 2 to 6, q is an integer having a value ranging from 1to 5, r is an integer having a value ranging from 2 to 10, R³ ishydrogen or methyl, and X denotes O, S, or —NR—, wherein R denotes analkyl group; each R² in structure (II) denotes methylene; a C₂₋₁₀n-alkylene group, such as a C₂₋₆ n-alkylene group; a C₂₋₆ branchedalkylene group, such as a C₃₋₆ branched alkylene group; a C₆₋₈cycloalkylene group; a C₆₋₁₄ alkylcycloalkylene, such as a C₆₋₁₀alkylcycloalkylene; a heterocyclic group, or—[(—CHR³—)_(s)—X—]_(q)—(—CHR³—)_(r)—; wherein R³, s, q, r, and X are asdefined above; m is a rational number having a value ranging from 0 to50, such as 0 to 10 or 1 to 10; n is an integer having a value rangingfrom 1 to 60; and p is an integer having a value ranging from 2 to 6.

Suitable polythioethers can be prepared by a number of methods. Incertain embodiments, (>n) moles of a compound having the structure (I)described earlier or a mixture of at least two different compoundshaving the structure (I), are reacted with (n) moles of a compoundhaving the structure (IIIa) and/or (IIIb), in the presence of acatalyst:—ROCH═CH₂   (IIIa)CH₂═CH—O—(—R²—O—)_(m)—CH═CH₂   (IIIb)

In formula (IIIa), R is a divalent hydrocarbon radical having from 2 to20 carbon atoms, wherein the hydrocarbon radical does not include afunctional group reactive with —SH, such as epoxy groups andethylenically unsaturated groups.

The compounds of formula (IIIa) are vinyl ethers. Specific examples ofsuitable vinyl ethers include, without limitation, ethyl vinyl ether,propyl vinyl ether, butyl vinyl ether, isobutyl vinyl ether, dodecylvinyl ether, octadecyl vinyl ether, ethylene glycol monovinyl ether,butanediol monovinyl ether, ethylene glycol butyl vinyl ether,triethylene glycol methyl vinyl ether, 2-ethylhexyl vinyl ether,cyclohexyl vinyl ether, tert-butyl vinyl ether, tert-amyl vinyl ether,diethylene glycol monovinyl ether, hexanediol monovinyl ether,aminopropyl vinyl ether, and 2-diethylaminoethyl vinyl ether.

In formula (IIIb), m is a rational number having a value ranging from 0to 50, such as 0 to 10 or 1 to 10, and R² is defined as in structure(II).

The compounds of structure (IIIb) are divinyl ethers. Divinyl etheritself (m=0) can be used. Other suitable divinyl ethers include thosecompounds having at least one oxyalkylene group, such as from 1 to 4oxyalkylene groups (i.e., those compounds in which m is an integer from1 to 4). In certain embodiments, m is an integer from 2 to 4. It is alsopossible to employ commercially available divinyl ether mixtures inproducing suitable polythioethers. Such mixtures are characterized by anon-integral average value for the number of alkoxy units per molecule.Thus, m in structure (IIIb) can also take on non-integral, rationalvalues between 0 and 50, such as between 1 and 10, or, in some cases,between 1 and 4, such as between 2 and 4.

Exemplary divinyl ethers include those compounds in which R² instructure (IIIb) is C₂₋₆ n-alkylene or C₂₋₆ branched alkylene, such asethylene glycol divinyl ether (EG-DVE) (R²=ethylene, m=1); butanedioldivinyl ether (BD-DVE) (R² butylene, m=1); hexanediol divinyl ether(HD-DVE) (R²=hexylene, m=1); diethylene glycol divinyl ether (DEG-DVE)(R²=ethylene, m=2); triethylene glycol divinyl ether (R²=ethylene, m=3);and tetraethylene glycol divinyl ether (R²=ethylene, m=4) andpolytetrahydrofuryl divinyl ether. In certain embodiments, the polyvinylether monomer can further comprise one or more pendent groups selectedfrom alkylene groups, hydroxyl groups, alkeneoxy groups, and aminegroups. Useful divinyl ether blends include “PLURIOL®” type blends suchas PLURIOL® E-200 divinyl ether (commercially available from BASF), forwhich R²=ethyl and m=3.8, as well as “DPE” polymeric blends such asDPE-2 and DPE-3 (commercially available from International SpecialtyProducts, Wayne, N.J.).

Useful divinyl ethers in which R² in structure (IIIb) is C₂₋₆ branchedalkylene can be prepared by reacting a polyhydroxy compound withacetylene. Exemplary compounds of this type include compounds in whichR² is an alkyl-substituted methylene group, such as —CH(CH₃)— or analkyl-substituted ethylene such as —CH₂CH(CH₃)—.

Other useful divinyl ethers include compounds in which R² in structure(IIIb) is polytetrahydrofuryl (poly-THF) or polyoxyalkylene, in somecases having an average of about 3 monomer units.

In some cases, trivinyl ether monomers, such as trimethylolpropanetrivinyl ether; tetrafunctional vinyl ether monomers such aspentaerythritol tetravinyl ether; and mixtures thereof, can be used.

The reaction between the compounds of structures (I) and (IIIa) and/or(IIIb) is sometimes catalyzed by a free radical catalyst. Suitable freeradical catalysts include azo compounds, for example azobisnitrilecompounds such as azo(bis)isobutyronitrile (AIBN); organic peroxidessuch as benzoyl peroxide and t-butyl peroxide; and similar free-radicalgenerators. The reaction can also be effected by irradiation withultraviolet light either with or without the use of a photosensitizer,such as benzophenone.

The particles of the present invention can be prepared by any suitabletechnique, including, for example, those described in the Examples. Incertain embodiments, the particles are obtained by preparing an aqueousdispersion of particles in water with an aminoplast resin, understirring. A catalyst may then be added and the dispersion heated to, forexample, a temperature of 50 to 80° C. Following a hold period thecompound reactive with the aminoplast resin, such as a polythiol, maythen be added to the aqueous dispersion. The catalyst in this techniqueis not particularly limited and may include, for example, acids, such ashydrochloric acid, sulfuric acid, p-toluene sulfonic acid, nitric acid,and the like, or inorganic salts showing acidity in aqueous solution,such as aluminum sulfate, alum (ammonium aluminum sulfate), etc.

The thus formed exterior surface coated particles may be separated bycentrifugal or vacuum filtration, and the wet cake obtained can then betreated, if necessary, with a hot air flow, for example, to obtain adried product.

As previously indicated, certain embodiments of the present inventionare directed to compositions, such as fuel-resistant compositions, thatcomprise the foregoing particles comprising an exterior surface coatedwith a thin coating. In certain embodiments, for example, the presentinvention is directed to sealant compositions, such as fuel-resistantsealant compositions, that comprise the foregoing lightweight particlescomprising an exterior surface coated with a thin coating. As usedherein, the term “sealant composition” refers to a composition that iscapable of producing a film that has the ability to resist atmosphericconditions, such as moisture and temperature and at least partiallyblock the transmission of materials, such as water, fuel, and otherliquid and gasses. In certain embodiments, the sealant compositions ofthe present invention are useful, e.g., as aerospace sealants andlinings for fuel tanks.

In certain embodiments, the lightweight particles described herein arepresent in the compositions of the present invention in an amount of atleast 0.1 percent by weight, such as at least 1 percent by weight, basedon the total weight of the composition. In certain embodiments, thelightweight particles are present in the coating compositions of thepresent invention in an amount of no more than 40 percent by weight,such as no more than 20 percent by weight, 10 percent by weight, or, insome cases, no more than 5 percent by weight, based on the total weightof the composition. The amount of lightweight particles in thecomposition can range between any combination of the recited valuesinclusive of the recited values.

The compositions, such as sealant compositions, in certain embodiments,comprise a filler comprising the previously described particles and afuel-resistant sulfur-containing polymer binder. As used herein, theterm “binder” refers to a continuous material in which particles of thepresent invention are dispersed. Binder materials suitable for use inthe sealant compositions of the present invention include, but are notlimited to, (1) polysulfide polyformal polymers, such as those describedin U.S. Pat. No. 2,466,963; (2) alkyl side chain containingpolythioether polymers, such as those described in U.S. Pat. No.4,366,307; (3) polythioethers having no oxygen in the polymericbackbone, such as is described in U.S. Pat. No. 4,609,762; (4)polythioethers according to structure (II) described earlier, such as isdescribed in U.S. Pat. Nos. 5,912,319 and 6,172,179; (5) epoxy-cappedpolythioethers, such as is described in United States Patent ApplicationPublication 2005/0010003; (6) vinyl-terminated polythioethers, such asis described in United States Patent Application Publication2006/0270796; (7) thioethers that are the reaction product of reactantscomprising: (a) an alpha, omega dihalo organic compound, (b) a metalhydrosulfide, and (c) a metal hydroxide, such as is described in U.S.patent application Ser. No. 12/108,782; and (8) isocyanate functionalpolythioether-polyurethanes and/or polythiourethanes and/oramine/hydroxy-functional polythioethers, such as is described in U.S.patent application Ser. No. 11/772,840.

In certain embodiments, the sealant compositions of the presentinvention also comprise a curing agent, i.e., crosslinking agent,comprising at least one or more of the following: polyolefins,polyacrylates, metal oxides, and polyepoxides, that are co-reactive withthe reactive functional groups of the sulfur-containing polymer binder.Specific exemplary curing agents include hydantoin diepoxide, diglycidylether of bisphenol-A such as EPON 828 (Resolution Performance Products,LLC), diglycidyl ether of bisphenol-F, Novolac-type epoxides such asDEN-40® (Dow Plastics), epoxidized unsaturated phenolic resins, dimeracid-based epoxy resins, acrylic and methacrylic polyol esters, andtriallylcyanurate (TAC).

Depending on the nature of the sulfur-containing polymer(s) binder usedin the sealant compositions of the present invention, the compositionmay, in certain embodiments, comprise 90 percent to 150 percent, such as95 percent to 125 percent, of the stoichiometric amount of the selectedcuring agent(s) based upon —SH equivalents.

In certain embodiments, the compositions of the invention comprise atleast one filler, aside from the particles of the present invention.Fillers may be added to compositions of the invention to impartdesirable physical properties such as, for example, to increase theimpact strength, to control the viscosity, or to modify the electricalproperties. Fillers useful in the curable compositions of the inventionfor aviation and aerospace applications include those commonly used inthe art, such as carbon black, calcium carbonate, silica, and polymerpowders. Exemplary fillers include Sipernat® D-13 hydrophobicprecipitated silica (Degussa), Winnofil® SPM precipitated calciumcarbonate (Solvay Chemicals), Cabosil® TS-270 (Cabot Corporation),titanium dioxide (DuPont), aluminum hydroxide, and Orgasol® 1002 D Nat 1ultrafine polyamide powder (Atofina Chemicals).

In certain embodiments, compositions of the invention comprise at leastone additive selected from the following: plasticizers, colorants, cureaccelerators, surfactants, adhesion promoters, thixotropic agents, fireretardants, and masking agents.

In certain embodiments, compositions of the invention comprise at leastone plasticizer. In certain embodiments, the plasticizer comprises atleast one of the following: phthalate esters, chlorinated paraffins, andhydrogenated terphenyls. Examples of useful plasticizers include HB-40®modified polyphenyl (Solutia, Inc.), and tung oil (Campbell & Co.). Incertain embodiments, the plasticizer comprises from 1% by weight to 40%by weight of the total weight of the composition. In other embodiments,the plasticizer comprises from 1% by weight to 8% by weight of the totalweight of the composition.

In other embodiments, such as where the composition comprises epoxycuring agents, compositions of the invention include at least one cureaccelerator or catalyst. In certain embodiments, the cure acceleratorcomprises at least one of the following organic amine catalysts:triethylamine (TEA), 1,8-diazabicyclo[5.4.0]undec-7-ene(DBU),2,4,6-tris(dimethylaminomethyl)phenol (DMP-30),1,1,3,3-tetramethylguanidine (TMG), carbamate paste (PRC-DeSotoInternational), and 1,4-diazabicylco[2.2.2]octane (DABCO) (AirProducts). In certain embodiments, the catalyst can be, for example,titanate TBT (DuPont).

In certain embodiments, compositions of the invention comprise one ormore adhesion promoters and coupling agents. Adhesion promoters andcoupling agents can enhance adhesion of the polymeric components of thecomposition to particulate additives, such as the particles of thepresent invention, as well as to substrate surfaces. Examples ofadhesion promoters include phenolics such as Methylon 75108 phenolicresin (Occidental Chemical Corp.), and organosilanes comprising epoxy,mercapto or amino functionalities such as Silquest A-187®(8-glycidoxypropyl trimethoxysilane) and Silquest A-1100®(8-aminopropyltrimethoxysilane) (OSi Specialties). Other useful adhesionpromoters include organic titanates such as, for example, Tyzor® tetran-butyl titanate (TBT) (Dupont), hydrolyzed silane (PRC-DeSotoInternational), and phenolic cook (PRC-DeSoto International).

In certain embodiments, compositions of the invention comprise at leastone thixotropic agent. A thixotropic agent may stabilize the viscosityof the composition in response to sheer stress. In certain embodiments,the thixotropic agent comprises at least one of fumed silica and carbonblack.

In certain embodiments, compositions of the invention comprise at leastone fire retardant. A fire retardant reduces the combustibility of thecured composition.

In still other embodiments, compositions of the invention include atleast one masking agent, such as pine fragrance or other scents, whichare useful in covering any undesirable low-level odor of the curablecomposition.

In certain embodiments, compositions of the invention further compriseat least one volatile organic solvent, such as isopropyl alcohol. Theorganic solvent may be included to reduce the viscosity of the curablecomposition during application and evaporates following application.

In certain embodiments, the compositions of the present inventioncomprise a colorant. As used herein, the term “colorant” means anysubstance that imparts color and/or other opacity and/or other visualeffect to the composition. The colorant can be added to the coating inany suitable form, such as discrete particles, dispersions, solutionsand/or flakes. A single colorant or a mixture of two or more colorantscan be used in the coatings of the present invention.

Example colorants include pigments, dyes and tints, such as those usedin the paint industry and/or listed in the Dry Color ManufacturersAssociation (DCMA), as well as special effect compositions. A colorantmay include, for example, a finely divided solid powder that isinsoluble but wettable under the conditions of use. A colorant can beorganic or inorganic and can be agglomerated or non-agglomerated.Colorants can be incorporated into the coatings by use of a grindvehicle, such as an acrylic grind vehicle, the use of which will befamiliar to one skilled in the art.

Example pigments and/or pigment compositions include, but are notlimited to, carbazole dioxazine crude pigment, azo, monoazo, disazo,naphthol AS, salt type (lakes), benzimidazolone, condensation, metalcomplex, isoindolinone, isoindoline and polycyclic phthalocyanine,quinacridone, perylene, perinone, diketopyrrolo pyrrole, thioindigo,anthraquinone, indanthrone, anthrapyrimidine, flavanthrone, pyranthrone,anthanthrone, dioxazine, triarylcarbonium, quinophthalone pigments,diketo pyrrolo pyrrole red (“DPPBO red”), titanium dioxide, carbon blackand mixtures thereof. The terms “pigment” and “colored filler” can beused interchangeably.

Example dyes include, but are not limited to, those that are solventand/or aqueous based such as pthalo green or blue, iron oxide, bismuthvanadate, anthraquinone, perylene, aluminum and quinacridone.

Example tints include, but are not limited to, pigments dispersed inwater-based or water miscible carriers such as AQUA-CHEM 896commercially available from Degussa, Inc., CHARISMA COLORANTS andMAXITONER INDUSTRIAL COLORANTS commercially available from AccurateDispersions division of Eastman Chemical, Inc.

As noted above, the colorant can be in the form of a dispersionincluding, but not limited to, a nanoparticle dispersion. Nanoparticledispersions can include one or more highly dispersed nanoparticlecolorants and/or colorant particles that produce a desired visible colorand/or opacity and/or visual effect. Nanoparticle dispersions caninclude colorants such as pigments or dyes having a particle size ofless than 150 nm, such as less than 70 nm, or less than 30 nm.Nanoparticles can be produced by milling stock organic or inorganicpigments with grinding media having a particle size of less than 0.5 mm.Example nanoparticle dispersions and methods for making them areidentified in U.S. Pat. No. 6,875,800 B2, which is incorporated hereinby reference. Nanoparticle dispersions can also be produced bycrystallization, precipitation, gas phase condensation, and chemicalattrition (i.e., partial dissolution). In order to minimizere-agglomeration of nanoparticles within the coating, a dispersion ofresin-coated nanoparticles can be used. As used herein, a “dispersion ofresin-coated nanoparticles” refers to a continuous phase in which isdispersed discreet “composite microparticles” that comprise ananoparticle and a resin coating on the nanoparticle. Exampledispersions of resin-coated nanoparticles and methods for making themare identified in United States Patent Application Publication2005-0287348 A1, filed Jun. 24, 2004, U.S. Provisional Application No.60/482,167 filed Jun. 24, 2003, and U.S. patent application Ser. No.11/337,062, filed Jan. 20, 2006, which is also incorporated herein byreference.

Example special effect compositions that may be used in the compositionsof the present invention include pigments and/or compositions thatproduce one or more appearance effects such as reflectance,pearlescence, metallic sheen, phosphorescence, fluorescence,photochromism, photosensitivity, thermochromism, goniochromism and/orcolor-change. Additional special effect compositions can provide otherperceptible properties, such as opacity or texture. In a non-limitingembodiment, special effect compositions can produce a color shift, suchthat the color of the coating changes when the coating is viewed atdifferent angles. Example color effect compositions are identified inU.S. Pat. No. 6,894,086, incorporated herein by reference. Additionalcolor effect compositions can include transparent coated mica and/orsynthetic mica, coated silica, coated alumina, a transparent liquidcrystal pigment, a liquid crystal coating, and/or any compositionwherein interference results from a refractive index differential withinthe material and not because of the refractive index differentialbetween the surface of the material and the air.

In general, the colorant can be present in any amount sufficient toimpart the desired visual and/or color effect. The colorant may comprisefrom 1 to 65 weight percent of the present compositions, such as from 3to 40 weight percent or 5 to 35 weight percent, with weight percentbased on the total weight of the compositions.

The compositions of the present invention can be applied to any of avariety of substrates. Common substrates to which the compositions ofthe present invention are applied can include titanium, stainless steel,aluminum, anodized, primed, organic coated and chromate coated formsthereof, epoxy, urethane, graphite, fiberglass composite, KEVLAR®,acrylics and polycarbonates.

The compositions of the present invention can be applied directly ontothe surface of a substrate or over an underlayer by any suitable coatingprocess known to those of ordinary skill in the art, for example, by dipcoating, direct roll coating, reverse roll coating, curtain coating,spray coating, brush coating, vacuum coating and combinations thereof.The method and apparatus for applying the composition to the substratemay be determined, at least in part, by the configuration and type ofsubstrate material.

Certain embodiments of the compositions of the present inventiondesirably are cured at ambient temperature and pressure, however theformulations generally can be cured at a temperature ranging from 0° C.to 100° C. or higher.

As previously indicated, certain embodiments of the present inventionare directed to “fuel-resistant” sealant compositions. As used herein,the term “fuel resistant” means that the compositions of the presentinvention, when applied to a substrate and cured, can provide a curedsealant that has a percent volume swell of not greater than 40%, in somecases not greater than 25%, in some cases not greater than 20% afterimmersion for one week at 140° F. (60° C.) and ambient pressure (1atmosphere) in jet reference fluid (JRF) type 1 according to methodssimilar to those described in ASTM D792 or AMS 3269a, incorporatedherein by reference. Jet reference fluid JRF type 1, as employed hereinfor determination of fuel resistance, has the following composition (seeAMS 2629, issued Jul. 1, 1989), §3.1.1 et seq., available from SAE(Society of Automotive Engineers, Warrendale, Pa.) (that is incorporatedherein by reference): herein by reference):

Toluene 28 ± 1% by volume Cyclohexane (technical) 34 ± 1% by volumeIsooctane 38 ± 1% by volume Tertiary dibutyl disulfide 1 ± 0.005% byvolume (doctor sweet)

In certain embodiments, the fuel-resistant compositions of the presentinvention have a very low specific gravity. In certain embodiments, thefuel-resistant compositions of the present invention have a specificgravity of less than 1.0, such as no more than 0.9, in some cases nomore than 0.85, and, in yet other cases, no more than 0.8 g/cm³.

In certain embodiments, sealant compositions of the present inventionalso have good low temperature flexibility as determined by knownmethods, for example, by the methods described in AMS (AerospaceMaterial Specification) 3267 §4.5.4.7, MIL-S (MilitarySpecification)-8802E §3.3.12 and MIL-S-29574, and by methods similar tothose described in ASTM (American Society for Testing and Materials)D522-88, which are incorporated herein by reference. Cured formulationshaving good low temperature flexibility are desirable in aerospaceapplications because the formulations are subjected to wide variationsin environmental conditions, such as temperature and pressure, andphysical conditions such as joint contraction and expansion andvibration. As used herein, the term “flexible”, when used with referenceto the properties of a composition, means that the cured composition hasan elongation as described in AMS 3269a of at least 80%.

Furthermore, the compositions of the present invention, when applied toa substrate and cured, provide a cured sealant having high tensilestrength. In other words, the cured sealant has a tensile strength of atleast 2000 mega Pascals when measured according to AS 5127/1a 7.7.

As should be apparent from the foregoing description, the presentinvention is also directed to methods for sealing an aperture utilizinga composition of the present invention. These methods comprise (a)applying a composition of the present invention to a surface to seal andaperture; and (b) allowing the composition to cure under, for example,ambient conditions.

Illustrating the invention are the following examples, which, however,are not to be considered as limiting the invention to their details.Unless otherwise indicated, all parts and percentages in the followingexamples, as well as throughout the specification, are by weight.

EXAMPLES Examples 1-3

Particles comprising an exterior surface coated with a thin coating wereprepared using the ingredients and amounts of Table 1.

TABLE 1 Amount (grams) Component Description Example 1 Example 2 Example3 1 Expancel 091 DE 80 6.0 6.0 6.0 d30¹ 2 De-ionized Water 551.8 551.8551.8 3 Cymel 303² 22.4 22.4 22.4 4 10% PTSA Solution³ 22.4 22.4 22.4 5Ethylenedioxy — 2.5 5.0 diethanethiol⁴ 6 Saturated Sodium 13 13 13Bicarbonate ¹Dry expanded thermoplastic hollow spheres commerciallyavailable from Akzo Nobel. ²Melamine-formaldehyde resin commerciallyavailable from Cytec Industries Inc. ³10% w/w para-toluene sulfuric acidin DI water. ⁴Commercially available from Sigma Aldrich.Components 1, 2, and 3 were charged to a 2 liter round bottom flaskequipped with a stirrer and a heating mantle. With stirring, component 4was added and the resultant mixture was warmed to 60° C. and held fortwo hours. Component 5, when used, was then added over two minutes, andthe mixture was then held an additional three hours at 60° C. Heat wasremoved and then component 6 was added. The mixture was then stirred for10 minutes. The solids were filtered in a buchner funnel, rinsed threetimes with clean water, and allowed to dry at ambient temperatureovernight, then 24 hours at 49° C. The powder was then sifted through a250 micron sieve.

Examples 4-7

Sealant compositions were prepared using the ingredients and amounts ofTable 2.

TABLE 2 Amount (grams) Component Description Example 4 Example 5 Example6 Example 7 1 Permapol 3.1e⁵ 152.54 152.54 152.54 152.54 2 CalciumCarbonate 44.8 44.8 44.8 44.8 3 Silquest A1100⁶ 2.75 2.75 2.75 2.75 4DABCO L33⁷ 0.94 0.94 0.94 0.94 5 Cabosil TS 720⁸ 1.3 1.3 1.3 1.3 6Example 1 Particles — 6 — — Example 2 Particles — — 6 — Example 3Particles — — — 6 7 Accelerator⁹ 50.8 50.8 50.8 50.8 ⁵Thiol-terminatedpolythioether commercially available from PRC-DeSoto International,Inc., Sylmar, California. ⁶γ-aminopropyltriethoxysilane commerciallyavailable from OSi Specialties. ⁷Catalyst commercially available fromAir Products and Chemicals, Inc. ⁸Treated fumed silica commerciallyavailable from Cabot Corp. ⁹2.31/1 mix of DEN 431(available from Dupont)and Epon 828 (available from Hexion specialty Chemical).

Each of the sealant compositions was rolled out to a uniform 0.125 inchthick sheet and allowed to cure at room temperature for two weeks. Theresulting sheets were tested for fuel-resistance, specific gravity,tensile strength, and elongation. Results are set forth in Table 3.

TABLE 3 Test Example 4 Example 5 Example 6 Example 7 % Swell - 1 week @101 85.4 77.3 77.4 140° F. in MEK¹⁰ % Swell - 1 week @ 19.9 12.7 12.411.9 140° F. in Jet reference fluid JRF type 1¹¹ Specific Gravity 1.011.03 1.01 1.05 (g/cm³) Tensile (mega 1716 1910 2040 2069 Pascal)¹²Elongation¹³ 85 88 82 92 ¹⁰Per AS5127/1A 7.5. ¹¹Per AMS 2929 type 1.¹²Per AS5127/1a 7.7. ¹³Per AMS 3269a.

Based on the foregoing results, it is believed to be a matter of routineformulation practice to achieve, using the lightweight particles of thepresent invention and a sulfur-containing polymer binder, a compositionthat has a specific gravity of less than 1.0, such as no more than 0.9,or, in some cases, no more than 0.85 g/cm³ and that, when applied to asubstrate and cured, results in a cured composition, such as a curedsealant, that has: (i) a percent volume swell of not greater than 40%,in some cases not greater than 25%, in some cases not greater than 20%after immersion for one week at 140° F. (60° C.) and ambient pressure injet reference fluid (JRF) type 1; (ii) an elongation of at least 80%when measured as described in AMS 3269a; and (iii) a tensile strength ofat least 2000 mega-Pascals when measured according to AS 5127/1a 7.7.Such compositions have not heretofore been achieved.

Examples 8-10

Particles described above were suspended in methyl ethyl ketone and thetime required for the particles to sink to the bottom of the containernoted. Results are set forth in Table 4.

TABLE 4 Example 8 9 10 Particles Example 1 Example 2 Example 3 Time(days) 13 27 40

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

We claim:
 1. A particle comprising an exterior surface having a coatingthereon, wherein the coating comprises a reaction product of reactantscomprising: (a) an aminoplast resin; and (b) a polythiol, wherein thepolythiol has the structure:HS—R¹—SH wherein R¹ is selected from a C₂₋₆ n-alkylene group; a C₃₋₆branched alkylene group having one or more pendent groups; a C₆₋₈cycloalkylene; C₆₋₁₀ alkylcycloalkylene group; heteroatomic substitutedanalogs of any of the foregoing, —[(—CHR³—)_(s)—O—]_(q)—(—CHR³—)_(r)—,and —[(—CHR³—)_(s)—S—]_(q)—(—CHR³—)_(r)—; each R³ is hydrogen; s is aninteger having a value ranging from 2 to 6; q is an integer having avalue ranging from 1 to 5; and r is an integer having a value rangingfrom 2 to
 10. 2. The particle of claim 1, wherein: R¹ is—[(—CHR³—)_(s)—O—]_(q)—(—CHR³—)_(r)-or-[(—CHR³—)_(s)—S—]_(q)—(—CHR³—)_(r)—,and q is an integer having a value ranging from 1 to
 3. 3. The particleof claim 2, wherein q is
 2. 4. The particle of claim 3, wherein s is 2and r is
 2. 5. A method of using the particle of claim 1, comprisingdispersing the particle in a composition comprising a thiol-functionalbinder.
 6. The method of claim 5, wherein the binder comprises athiol-functional sulfur-containing polymer.
 7. The method of claim 5,wherein the composition, after the dispersing step, has a specificgravity of less than 1.0 g/cm³.
 8. The particle of claim 1, wherein thecoating is substantially continuous from 70 percent to 100 percent ofthe exterior surface.
 9. The particle of claim 1, wherein the polythiolcomprises a polythioether.
 10. The particle of claim 1, wherein theparticle comprises a thermally expandable capsule.
 11. The particle ofclaim 10, wherein the thermally expandable capsule comprises a thermallyexpandable thermoplastic capsule.
 12. The particle of claim 1, whereinthe particle comprises a polystyrene foam, a polyacrylate microsphere, apolyolefin microsphere, a silica microsphere, an alumina/silicamicrosphere, an aluminum silicate microsphere, a calciumcarbonate-coated polyvinylidene copolymer microsphere, or a thermallyexpandable capsule.
 13. A composition comprising: a plurality ofparticles of claim 1; and a sulfur-containing polymer.
 14. Thecomposition of claim 13, wherein the sulfur-containing polymer comprisesa thiol-functional polythioether.
 15. The composition of claim 13,comprising a curing agent.
 16. The composition of claim 15, wherein thecuring agent comprises an epoxide.
 17. The composition of claim 13,formulated as a sealant having a specific gravity less than 1.0 g/cm.18. A cured sealant, comprising the composition of claim
 13. 19. Thecured sealant of claim 18, characterized by a specific gravity less than1.0 g/cm³.
 20. The cured sealant of claim 18, wherein the cured sealantexhibits: a percent volume swell of not greater than 40% after immersionfor one week at 140° F. (60° C.) and ambient pressure in Jet ReferenceFuel Type 1; an elongation of at least 80% when measured as described inAMS 3269a; and a tensile strength of at least 2000 mega-Pascals whenmeasured according to AS 5127/1a7.7.
 21. The particle of claim 1,wherein the coating is characterized by a thickness less than 25 μm. 22.The particle of claim 1, wherein the coating is characterized by athickness less than 500 nm.
 23. The particle of claim 1, wherein theaminoplast resin comprises a highly alkylated low imino aminoplastresins characterized by a degree of polymerization less than 3.75. 24.The particle of claim 1, wherein the polythiol comprises a polythiolselected from 1,2-ethanedithiol, 1,3-propanedithiol, 1,4-butanedithiol,1,5-pentanedithiol, 1,6-hexanedithiol, 1,2-propoanedithiol,1,3-butanedithol, 2,3-butanedithiol, 1,3-pentanedithiol,1,3-dithiol-3-methylbutane, dipentenedimercaptan,ethylcyclohexyldithiol, dimercaptodiethylsulfide, dimercaptodioxaoctane,1,5-dimercapto-3-oxapentane, and a combination of any of the foregoing.25. The particle of claim 1, wherein the polythiol comprises a polythiolselected from HS—CH₂CH(—CH₃)—S—CH₂CH₂—SH, HS—CH(—CH₃)CH₂—S—CH₂CH₂—SH,HS—CH₂CH(—CH₃)—S—CH(—CH₃)CH₂—SH, HS—CH(—CH₃)CH_(2—S—CH) ₂CH(—CH₃)—SH,and a combination of any of the foregoing.
 26. A particle comprising anexterior surface having a coating thereon, wherein the coatingcomprises: (a) a first film comprising an aminoplast resin reacted withthe exterior surface; and (b) a second film comprising a polythiolreacted with the first film, wherein the polythiol has the structure:HS—R¹—SH wherein R¹ is selected from a C₂₋₆ n-alkylene group; a C₃₋₆branched alkylene group having one or more pendent groups; a C₆₋₈cycloalkylene; C₆₋₁₀ alkylcycloalkylene group; heteroatomic substitutedanalogs of any of the foregoing, —[(—CHR³—)_(s)—O—]_(q)—(—CHR³—)_(r)—,and —[(—CHR³—)_(s)—S—]_(q)—(—CHR³—)_(r)—; each R³ is hydrogen; s is aninteger having a value ranging from 2 to 6; q is an integer having avalue ranging from 1 to 5; and r is an integer having a value rangingfrom 2 to
 10. 27. A particle comprising an exterior surface having acoating thereon, wherein the coating comprises: (a) the reaction productof an aminoplast resin and the exterior surface; and (b) the reactionproduct of a polythiol with (a), wherein the polythiol has thestructure:HS—R¹—SH wherein R¹ is selected from a C₂₋₆ n-alkylene group; a C₃₋₆branched alkylene group having one or more pendent groups; a C₆₋₈cycloalkylene; C₆₋₁₀ alkylcycloalkylene group; heteroatomic substitutedanalogs of any of the foregoing, —[(—CHR³—)_(s)—O—]_(q)—(—CHR³—)_(r)—,and —[(—CHR³—)_(s)—S—]_(q)—(—CHR³—)_(r)—; each R³ is hydrogen; s is aninteger having a value ranging from 2 to 6; q is an integer having avalue ranging from 1 to 5; and r is an integer having a value rangingfrom 2 to 10.